1. Field of the Invention
The present invention relates generally to an articulable joint which will permit angular translation between two jointed sections and transmission of torque along the length of the jointed sections. The joint may be incorporated into a joint assembly which is useful for transferring eccentric rotational energy and motion into concentric rotational energy and motion. In a described embodiment, the invention has application as a coupling to be incorporated into a downhole drilling device. In particular, the coupling is applied to connect a motor section to a drill bit shaft.
2. Description of Related Art
In many mechanical devices, there is a need to interconnect adjoining members so that axial rotation can be transferred from one member to the other while angular translation of one member with respect to the other is permitted at the connection point. Articulable joints and joint assemblies have been known which are operable to provide this type of flexible motion between the connected members. Perhaps the most well known designs of this type are universal joints which consist of forked arm sections joined with a spider element, the link pins of which engage eyelets in the arms. However, these designs are not generally sufficient for transfer of high torque and thrust loads.
Joint assemblies which incorporate such articulable joints are often employed when it is desired to convert rotation from an eccentrically moving member to a concentrically rotatable member. For example, rotation of the drill bit is imparted by a downhole motor assembly. The motor assembly itself typically includes a rotor portion which moves in an eccentrically rotating manner within a stationary stator section. However, the motor assembly must be coupled or otherwise associated with the bit such that the eccentric motion of the rotor imparts concentric motion to the bit. Many contemporary drill strings employ what is commonly known as a lobe coupling to perform this function. This type of connector utilizes a pair of universal joints which connect a straight connecting rod to the rotor and to the bit shaft. Unfortunately, the universal joint sections are designed to transfer largely torsional loads and are relatively poorly suited to handle significant hydraulic thrust-type loading. As a result, lobe couplings must typically be repaired or replaced after 70-80 hours of operation. The expenses associated with removing the drill string from the well and downtime for the well are often significant.
Alternative designs have been developed in attempts to improve coupling life. Connector designs are known in which a flexible rod extends between the rotor and the shaft for translating the torque and eccentric motion of the rotor to the concentrically supported bit drive shaft. In one such design, the rod has an upset section on each end, and upper and lower connections connect the upset sections of the rod to the rotor and to the shaft. The connections are non-integral to the rod and can be made from a different material from the rod.
A design is also known wherein a torsion bar is used to connect the rotor and the shaft and for translating the rotation and eccentric motion of the rotor to the concentric rotation of the bit shaft. Connection between the torsion bar and the rotor is disposed above the area of contact of helical teeth of the rotor with helical teeth of the stator. Preferably, for connecting the rotor to the torsion bar, the rotor is provided with a recess tapering upwardly and a through-slot disposed above this recess perpendicularly to its axis to communicate therewith. The torsion bar has a conical portion corresponding to the tapered recess of the rotor and terminating in a tailpiece of rectangular configuration received by the slot and having in cross-section dimensions corresponding to the dimensions of this slot.
Clearly, a joint and joint assembly design which is durable and strong is desirable. In addition, a drill string coupling is needed which is sufficiently sturdy and efficient for effective transfer of both torsional loading and hydraulic thrust-type loading.